Devices in computer systems typically communicate with each other using a bus. In many computer systems, when a sending device outputs data on a bus, it simultaneously outputs a clock or "strobe" that informs a receiving device when the data on the bus can be read. Buses that are accompanied by a strobe are referred to as "synchronous strobe buses."
FIG. 1 is a block diagram illustrating communication using a synchronous strobe bus between two computer devices: a chipset 10 and a graphics controller 12. A data bus 14 exchanges data between devices 10 and 12. Data bus 14 is typically a bidirectional multi-bit bus (e.g., a 16-bit data bus). A strobe line 16 carries a strobe signal from device 10 to device 12, and vice versa. The strobe signal indicates when data on data bus 14 can be read by device 10 or device 12.
FIG. 2 is a timing diagram illustrating data 20 on data bus 14 in relation to a strobe signal 22 on strobe line 16. The data 20 between a first transition of data bus 14 at time To and a second transition at time T.sub.2 is "D.sub.0 " (i.e., the bits on data bus 14, when read by devices 10 or 12, form D.sub.0). Between the second transition at time T.sub.2 and a third transition at time T.sub.4, data 20 is "D.sub.1 ".
Devices 10 and 12 read data 20 on data bus 14 when strobe signal 22 transitions from high to low, or from low to high. Therefore, in FIG. 2, data 20 is read at time T.sub.1 and at time T.sub.3.
When implementing synchronous strobe buses, there is a need to have an adequate setup and hold time. The setup time, "T.sub.su " (between T.sub.0 and T.sub.1 and between T.sub.2 and T.sub.3 in FIG. 2), is the time from when data 20 is initially valid (i.e., can be read by a receiving device) to the time strobe signal 22 transitions. The hold time "T.sub.hold " (between T.sub.1 and T.sub.2 in FIG. 2), is the time from when strobe signal 22 transitions until the time data 20 is no longer valid.
The setup time and hold time for a given window are inversely related. Specifically, when the setup time is increased, the hold time is decreased, and vice versa. In order to optimally position the strobe signal 22 relative to the data 20 signal, it is desirable for the setup time to equal the hold time. This is referred to as "balancing" the setup time and hold time. A balanced setup and hold time provides for a maximum error margin on either side of a transitioning strobe signal 22.
However, the setup time and hold time can vary depending on the number of bits that are toggled on data bus 14, and the type of toggles (i.e. either a 1 to 0 toggle or a 0 to 1 toggle). For example, because of incremental delay induced by simultaneous switching outputs, as more bits are toggled on data bus 14, it takes longer for all the bits on data bus 14 to be in condition to be read by a receiving device. Therefore, when the number of bits toggled is increased, the setup time is decreased and the hold time is increased.
A problem with prior art devices that include a synchronous strobe bus is that the strobe signal 22 is typically adjusted so that the setup time is balanced with the hold time when one bit of data 20 is toggled. However, when more than one bit is toggled, the setup time is decreased, and the setup time and hold time are no longer balanced. Because the setup time and hold time in prior art devices are not always in balance, these devices must either maintain a larger setup and hold time or contend with a smaller margin of error. However, it is desirable to reduce the setup and hold time as much as possible because the frequency of the devices can be increased as the setup and hold times are decreased.
Based on the foregoing, there is a need for a method and apparatus for operating a synchronous strobe bus with a reduced setup and hold time compared to the prior art.